Anthropogenic CO₂ invasion into the northeast Pacific based on concurrent δ¹³CDIC and nutrient profiles from the California Current

Abstract:

The stable isotopic signature of dissolved inorganic carbon (δ¹³CDIC) in the northeast Pacific Ocean is lower in near-surface waters by 1.1‰ relative to values predicted from global oceanic trends of δ¹³CDIC versus nutrients. A combination of anthropogenic carbon uptake from the atmosphere and thermodynamic, air-sea gas exchange processes in different water mass source areas account for the isotopic depletion. Here we evaluate the efficacy of using a concurrent nutrient-δ¹³C strategy to separate these two effects, with the goal of improving estimates of anthropogenic carbon uptake over the course of the Industrial Revolution. In depth profiles from the sea surface to 2500 m at four stations across the California Current (42ºN), nitrate, rather than phosphate, is best correlated to δ¹³CDIC providing the best choice for this experiment. On the basis of an assumption of no anthropogenic carbon in North Pacific Deep Waters between 1000–2500 m depth (potential densities, σθ ~ 27.3–27.7), the “anthropogenic – preanthropogenic” carbon isotope shift (∆δ¹³Ca–p) in near-surface waters of the northeast Pacific is inferred to be -0.62 ± 0.17‰, while the thermodynamic air-sea gas exchange signature is estimated at -0.48 ± 0.17‰. Values of ∆δ¹³Ca–p (similar to the regional patterns of ∆¹4C and Tritium penetration) approach zero for σθ > 26.8, indicating little penetration of anthropogenic carbon into the North pacific Intermediate Water or the upper North pacific Deep Water. Our results suggest an upper North Pacific sink of anthropogenic carbon over the past ~200 years that is ~40% greater than that estimated for the interval between ~1970 and ~1990 by Quay et al. [1992]. Our estimate of the North Pacific inventory of anthropogenic carbon, added to published estimates from the North Atlantic and Indian Ocean, is smaller than model predictions of the total carbon sink, suggesting that a significant portion of anthropogenic carbon enters the deep sea via the Southern Ocean.